Armed pro-Russian separatists seized more buildings in eastern Ukraine on Monday, expanding their control after the government failed to follow through on threatened military crackdown leaving Moscow's partisans essentially unopposed.

European foreign ministers agreed to widen sanctions against Moscow and the White House said Washington was seeking ways to impose more "costs" on Russia, for what Kyiv and its Western friends call a Russian plot to dismember Ukraine.

Rebels in the town of Slaviansk, where the authorities failed to follow through with their announced "anti-terrorist" operation, called for Russian President Vladimir Putin's help.

Ukraine's interim president Oleksander Turchinov said on Monday the offensive against the rebels would still go ahead. But in a sign of discord behind the scenes in Kyiv, he sacked the state security chief in charge of the operation.

USS Donald Cook, stationed in the Black Sea near Romania, was buzzed by a Russian fighter jet on Saturday, U.S. officials said Monday. (Emrah Gurel/AP)

In Donetsk, rebels holed up in the administrative headquarters of a province that is home to 10 per cent of Ukraine's population said they planned to seize control of infrastructure and the levers of state power. They have declared an independent "People's Republic of Donetsk" and sought Putin's protection if they are attacked.

Rebels have also seized buildings in around 10 other towns and cities across other eastern provinces which form the heartland of Ukraine's heavy industry.

In a bid to undercut the rebels' demands, Turchinov held out the prospect of a countrywide referendum on the future shape of the Ukrainian state. Pro-Russian secessionists want separate referendums in their regions, which Kyiv says is illegal.

The uprising in eastern Ukraine began eight days ago but has accelerated sharply in the past 48 hours, with separatists seizing ever more buildings, including arsenals filled with weapons. They have met little opposition.

Kiev says the separatists are organized by Moscow, seeking to repeat the seizure of the Crimea region, which Moscow occupied and annexed last month.

Russian response appears co-ordinated

Russia says the armed men are all locals acting on their own, but Western officials say the uprising is too well-coordinated to be entirely spontaneous, and bears too many similarities to the Russian operation in Crimea.

'I don't think denials of Russian involvement have a shred of credibility.'- British Foreign Minister William Hague

"I don't think denials of Russian involvement have a shred of credibility," British Foreign Minister William Hague said, before a meeting with EU counterparts.

Hague later announced that the ministers had agreed to expand a list of Russians barred from travelling or doing business in the EU. Work would begin to come up with new names for the sanctions list, Hague said.

In Washington, White House spokesman Jay Carney said U.S. President Barak Obama would speak to Putin by phone later on Monday. Washington is also planning to expand its sanctions list. Russia has so far shrugged off targeted sanctions.

Moscow says it has the right to intervene to protect Russian speakers in Ukraine, and has portrayed the people of the east as under threat from gangs of Ukrainian-speaking "fascists". NATO says Russia has tens of thousands of troops massed on the frontier, able to capture eastern Ukraine within days.

Ultimatum expires

Turchinov had threatened to launch a military crackdown by 9 a.m., but as the deadline expired there was no sign of any action in Slaviansk. A rebel leader, in an appeal issued through journalists, asked Putin to "help us as much as you can."

A pro-Russian protester gestures at a barricade in front of the seized office of the SBU state security service in Luhansk, in eastern Ukraine on Monday. Towns in eastern Ukraine on Monday braced for military action from government forces after Kyiv gave pro-Russian separatists a morning deadline to disarm and end their occupation of state buildings or face a major 'anti-terrorist' operation. (Shamil Zhumatov/Reuters)

The Kremlin said the Russian president was listening.

"Unfortunately, there's a great many such appeals coming from the Eastern Ukrainian regions addressed directly to Putin to intervene in this or that form," spokesman Dmitry Peskov said. "The president is watching the developments in Eastern Ukraine with great concern."

Also in Slaviansk, about 150 kilometres from the Russian border, a small airfield which was occupied by Ukrainian air force planes on Sunday was empty on Monday and pro-separatist forces said they were now in control of it.

Eastern Ukraine seems to be rapidly spinning out of the control of the central government. The governor of Donetsk, a multi-millionaire appointed by Kyiv, has not been seen since April 11. A man calling himself Donetsk's new police chief has appeared wearing the orange and black separatist ribbon.

The Ukrainian defence ministry acknowledged that it has had difficulty mobilizing the armed forces in the east, where some units have been blockaded in by rebellious locals.

"On some occasions we have lost the information war and there have been blockades of our units. People don't understand why they are coming," said acting Defence Minister Mykhailo Koval. He said 26 members of a reconnaissance unit had been blockaded for the past day and a half in Slaviansk.